How to make a PDF smaller

Why some PDFs are so large

Text is cheap. A hundred pages of typed text can fit in a few hundred kilobytes, because each letter is stored as a character, not as a picture. Images are what make a PDF heavy. A single page scanned in colour at 600 dots per inch can take several megabytes, and a phone photo dropped into a document often carries far more detail than a screen or a printer will ever show.

• Scanned documents: every page is one large image, even when it only shows text.
• Photos placed in reports, catalogues and portfolios at their full camera resolution.
• The same logo or background image stored again on every page.
• Embedded fonts, which add a little to every file but rarely explain a huge one.

What compression actually does

PDF compression mostly means re-saving the images at a lower resolution and a higher JPEG compression, so they keep enough detail to read on screen and print on an office printer, and drop the rest. A good compressor also stores repeated images once and writes the file in a more compact structure. Text and vector drawings, such as charts and line art, are left as they are, so they stay sharp.

That is why results vary so much. A 40 MB scan can come down to 4 MB, while a 2 MB text-only report may hardly change, because there is nothing in it to shrink. Be wary of any tool that promises a fixed percentage before it has seen your file.

How to compress a PDF in your browser

1. Open Compress PDF on all-pdf.app and choose your file, or drop it onto the box.
2. Pick a level. Balanced suits most documents; Light keeps more detail for photos you will print; Strong gives the smallest file for on-screen reading.
3. Select Compress PDF and wait while the images are re-saved.
4. Compare the before and after sizes, and open the result to check that small print is still readable before you send it.

The work happens on your own device: we never upload your file. If the compressed version would come out larger than the original, you get the original back unchanged rather than a bigger file.

Common size limits to aim for

Email adds roughly a third to the size of an attachment in transit, so a file just under the limit can still be refused. Aim for a comfortable margin below it.

When compression is not enough

• Split it: Split PDF turns one large file into several smaller ones you can send separately.
• Remove what is not needed: Delete pages drops appendices, blank pages and duplicate scans.
• Rescan smarter: 150 to 200 dots per inch in greyscale is plenty for text documents, and many times smaller than 600 dpi colour.
• Export smaller at the source: in Word, choose Save As, PDF, then "Minimize size (publishing online)" before saving.
• On a Mac, Preview's "Reduce File Size" filter is quick, but it often makes pages blurry, so check the result.

Avoid turning the pages into JPG images just to make the file smaller. The text stops being selectable and searchable, and the file is often no smaller than a properly compressed PDF.